Implementing OCL as a Database Query Language

The approach of model-driven software development sketches the vision of the next big step in raising the level of abstraction and flexibility of programming tools. The key expectation behind MDA is achieving a productivity gain through the automating software construction based on models. In this paper we focus on one of possible model application, namely executable models. If models are to be executable, precise semantics is inevitable. Furthermore, executable models could blur the distinction between modelling and programming, since they would facilitate automatic production of executable code. This requires the presence of sophisticated transformation tools encapsulating the knowledge on particular target platform technologies. UML elements of Actions and Activities make it possible to represent programs similar to those in common programming languages but data processing requires querying capability as well. Seamless integration of UML [2] as a programming language and OCL [1] as a query language was done in the following steps: 1. Data structures are modelled using UML Classes package. However, since the standard is not defining any kind of global containers for database objects (like for instance user’s variables) we assume that each model must contain at least one class marked with <> stereotype that will be a definition for root database objects. Attributes of this class will be treated as global application variables from which all queries may begin. We also keep extension-starting queries which may use class names representing a collection of all their instances. 2. UML Actions and UML Activities are considered to be the imperative part of the language. They cover all procedural constructs, loops, conditional statements, variable declarations, exceptions throwing and catching, etc. Since UML does not define any kind of textual syntax for these elements and there are no modelling tools capable of creating such models yet, we decided to introduce our own textual syntax for them. This gives us possibility of describing programs which may be then converted to UML models and stored as instances of a metamodel. 3. The metamodel is a result of integration UML and OCL metamodels as described in OCL specification and implemented by MDT-OCL Eclipse plug-ins [3].